Since the end of World War II, uranium and plutonium have become infamous household words synonymous with the potential for mass destruction. Nine countries now openly possess nuclear weapons and others are thought to have covert programs in various stages of development. Radiological weapons (i.e., “dirty bombs”), while less destructive than a nuclear weapon, could also cause substantial economic damage, endanger the public health, and lead to significant environmental contamination. There is thus an urgent demand for faster, more portable detection methods, including those that can be used to sense species, such as the high valent actinide cations, uranyl, neptunyl, and plutonyl, which are likely to be present on a relatively large scale under a variety of less-well-controlled conditions (i.e., following a spill or an untoward release).
The actinides (An) are easily hydrolyzed acidic metal ions that form strong complexes with common chelating agents (Clark et al., Chem Rev 95:25-48, 1995). The early actinides, between U and Am, are known for their diverse redox chemistry. The penta- and hexavalent oxidation states are generally the most common, especially for Np and Pu, wherein these actinides, like U(VI), exist as linear dioxocations. It is thus these species that are the most important in terms of sensor development for radioactive actinide cations.
To date, the problem of generating colorimetric actinide sensors, small molecules or receptors or constructs derived from them that change color when exposed to these species, has received relatively little attention. Two indicators that have been extensively studied are 2,2′-(1,8-dihydroxy-3,6-disulfonaphtylene-2,7-bisazo)-bisbenzenarsonic acid (AzIII) (Rohwer et al., Anal Chim Acta 341:263-268, 1997) and BrPADAP (Suresh et al., Spectrochim Acta A 58:341-347, 2002). These dyes have low limits of detection: 46 ppb for AzIII in aqueous media and 200 ppb for BrPADAP in ethanol. However, both suffer from drawbacks that make them less-than-ideal candidates for actinide detection. For instance, AzIII has a low selectivity for the actinides and, in fact, has a lower detection limit for the lanthanides (Ln) than for UO22+ (e.g., 20 ppb with Gd(III)). This is problematic since the lighter lanthanides are produced in fission events and could act as interferants (Roberto et al., Report of the Basic Energy Sciences Workshop on Basic Research Needs for Advanced Nuclear Energy Systems; Office of Basic Energy Sciences, DOE: October, 2006, p 440). To avoid detection of the Ln rather than An cations, a pre-purification step to remove the lanthanides is generally necessary (Collins et al., Anal Chim Acta 436:181-189, 2001). BrPADAP suffers from the fact that it complexes Th(IV) strongly and displays reduced accuracy for uranium and plutonium in the presence of this cation Suresh et al., Spectrochim Acta A 58:341-347, 2002). Furthermore, this dye is not water-soluble and gives rise to only a slight color change upon metal complexation. Both AzIII and BrPADAP are difficult to functionalize, which further limits the scope of their utility.
Another potential colorimetric actinide sensor was reported by Kubo et al., who described the synthesis of a calix[6]arene functionalized with a single indoaniline chromophore (Kubo et al., J Chem Soc, Chem Commun 1725-1726, 1994). In the presence of UO2(OAc)2, a bathochromic shift was observed (from 628 to 687 nm) that was not seen in the presence of Cs+, Li+, Sr2+, Na+, Ba,2+, or K+. To date, this system has not been functionalized for attachment to a solid support.
In work focused more on complexation than sensing, Taran and coworkers developed a combinatorial approach to the synthesis of uranyl receptors (Sawicki et al., Chem Euro J 11:3689-3697, 2005). These researchers screened 96 potential uranyl complexing agents using a competitive displacement strategy. Analysis via fluorescence titrations confirmed that the best system obtained in this way could be used to detect uranyl concentrations of less than 10−11 M; selectivity over alkali and alkali earth cations, but not Fe3+, was also observed. This system, however, did not permit direct detection via an easy-to-see color change.
In light of the above, there is a need for improved spectrometric actinide sensors. Particularly advantageous would be systems that could be attached to solid supports because this permits the conversion of molecular entities that display a spectrometric response in the presence of actinides into actual sensing devices. These and other needs are addressed herein through the production of certain functionalized expanded porphyrins, e.g., β-pyrrolic-, meso-, and β-pyrrolic and meso-substituted isoamethyrins.